1. Technical Field
The present invention relates to data communications and more particularly, to devices and methods for generating an envelope waveform for data signals received over telephone lines.
2. Background Art
Local area networks use a network cable or other network media to link nodes (e.g., workstations, PCs and switches) to the network. Each local area network architecture uses a media access control (MAC) layer that enables a network interface device at each node to share access to the media. Conventional local area network architectures use media access controllers operating according to half-duplex or full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol using prescribed network medium.
With the increased use of computers in the home, a need has developed for networking these computers. However, connecting multiple PCs and peripherals in the home typically requires installing special cabling and setting up Ethernet repeaters to link the devices.
Recently, efforts have been made to link computers in the home using Plain Old Telephone Service (POTS) line wiring instead of established local area network media. Such an arrangement, referred to herein as a home network environment, provides the advantage that existing telephone wiring in a home may be used to implement a home network. However, telephone lines are inherently noisy due to spurious noise caused by electrical devices in the home, for example, dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer from turn-on transients due to on-hook and off-hook and noise pulses from the standard POTS telephones. Further, the wiring topology of homes is variable, which makes data transmission signaling more complex.
One way for a network station to avoid problems with noise in the telephone network is to detect the arrival time of pulses used to convey digital information. For example, a band limited waveform, e.g., between 5.5 MHz and 9.5 MHz, can be used to effectively reject POTS signaling and all out of band noise and harmonic power. The envelope of the band limited waveform can then be generated and supplied to a slicing circuit having a threshold level selected to identify the arrival position of a received pulse. When the envelope crosses the threshold level, the slicing circuit detects the arrival position of the pulse. The detected signal may then be effectively sliced with an adaptive slice level designed to track the ambient noise level.
Previous attempts to generate the envelope of an incoming signal typically require amplifiers requiring high power, e.g., +5 volts and -5 volts, Schottky diodes and other discrete elements, such as relatively large capacitors. These elements are not practical in a low power CMOS device that exhibits high speed operation due to high power requirements, space constraints and general unreliability of such elements implemented in CMOS.